This invention relates to a heat-insulating paper container having a foamed layer of a thermoplastic synthetic resin film on the outer wall surface of the body member. More particularly, the invention relates to a paper container in which the thickness of the foamed heat-insulating layer in one area differs from that in another area.
Several types of heat-insulating containers have been used commercially to pack hot liquids. A polystyrene foam heat-insulating container is one example. This is produced by a process comprising the addition of a foaming agent to polystyrene, casting the mixture into a mold, heating the resin under pressure to foam it, and removing the shaped article from the mold. The container thus produced has outstanding heat-insulating properties but, on the other hand, it needs reconsiderations from the viewpoint of saving petroleum resources or increasing the efficiency of incinerating waste containers. As a further problem, a curved printing press or a stamp printing press is required to print on the outer surfaces of polystyrene foam heat-insulating containers since printing can only be effected after individual cups have been shaped. Thus, the polystyrene foam containers have suffered the disadvantage of low printability.
Another type of heat-insulating paper containers are such that the outer side wall of the body member is entirely surrounded by a corrugated paper jacket. The process of manufacturing such container involves additional steps of forming the corrugated paper jacket and bonding it to the outer surface of the side wall of the body member. This type of containers have various other defects. First, letters, figures or other symbols are printed on the corrugated surface and the resulting deformed letters or patterns do not have good aesthetic appeal to consumers. Secondly, the jacket is bonded to the side wall of the body member in such a manner that only the valley ridges contact the side wall and the bond between the jacket and the side wall is so weak that the two will easily separate. Thirdly, containers having the corrugated jacket are not suitable for nesting and, hence, need a large area of storage.
Still another type of heat-insulating paper containers have a "dual structure" wherein an inner cup is given a different taper than an outer cup to form a heat-insulating air layer in the space between the two cups. The two cups are made integral by curling their respective upper edges into a brim. Containers of this type have very good printability since the outer cup has a smooth-surfaced outer side wall. On the other hand, the two cups are prone to separation. A further defect is that the dual structure generally contributes to high manufacturing cost.
Unexamined Published Japanese Patent Application (kokai) Sho 57-110439 teaches a heat-insulating paper container consisting of a body member and a bottom panel member, characterized in that at least one surface of the body member is coated or laminated with a foamed heat-insulating layer of a thermoplastic synthetic resin film whereas the other surface of the body member is coated or laminated with a thermoplastic synthetic resin film, a foamed heat-insulating layer of said thermoplastic synthetic resin film or an aluminum foil. As already taught by Examined Japanese Patent Publication (kokoku) Sho 48-32283, the water in the paper is vaporized upon heating, causing the thermoplastic synthetic resin film on the surface to foam. The container under consideration has the advantage that it exhibits fairly good heat-insulating property and that it can be manufactured at low cost by a simple process.
This approach, however, has its own limitations; first, the thermoplastic synthetic resin film will not foam adequately if the water content in the paper is low; second, high water content is advantageous for the purpose of film foaming but, on the other hand, the mechanical strength of the container may deteriorate; thirdly, even if successful foaming is done, the thickness of the foam layer is uniform and cannot be adjusted from one portion to another.